1. Field of the Invention
The present invention relates to a magnetic recording medium, a method for producing the same and a method for forming a film by a plasma CVD (chemical vapor deposition). In particular, the present invention relates to a magnetic recording medium which is used in an audio or video equipment, a computer, and the like, and has a back coating layer on a surface of a nonmagnetic substrate reverse to a magnetic recording layer.
2. Description of the Related Art
In these years, a magnetic recording equipment is required to have a large capacity, achieve high speed reading and writing, high picture and sound quality, and have a small size and a light weight. To satisfy such requirements, it is inevitable for a magnetic recording medium to achieve a high density recording. To this end, instead of a conventional powder coating type magnetic recording medium having a magnetic layer which comprises a binder and magnetic powder dispersed in said binder, a ferromagnetic metal thin film magnetic recording medium has been actively developed and practically used, since it has a larger residual magnetic flux density (Br) and coercive force (Hc) and a thinner magnetic layer, and is more suitable for ultra-smoothing of a magnetic layer surface than the powder coating type one.
With the increase of surface smoothness of the magnetic layer, a coefficient of friction of the magnetic recording medium increases, so that running stability and durability tend to be worsened. Then, it is proposed to provide a back coating layer having specific surface roughness on a surface of a nonmagnetic substrate reverse to the magnetic layer.
The surface roughness of the back coating layer, in particular that of the ferromagnetic metal thin film magnetic recording medium should be as small as possible to decrease a so-called back transfer, that is, shape transfer of unevenness of the back coating layer to the magnetic layer surface in a winding up step or a heat treating step, and to prevent deterioration of electromagnetic conversion characteristics.
To this end, it is proposed to decrease a particle size of a filler to be contained in the back coating layer, or to add an excessive amount of a dispersant or use a binder having a relatively large amount of a polar group such as a sulfonate in a molecule for improving dispersibility of the filler. However, these measures result in decrease of film strength of the back coating layer, so that the back coating layer tends to be scraped off or a recording tape tends to be folded during tape running.
In addition, worn debris from the back coating layer is transferred to the magnetic layer surface during tape winding and may clog a magnetic head, increase the number of drop-outs or decrease output signal level. When the magnetic recording medium is stored for a long time, it suffers from blocking, namely adhesion of the magnetic layer and the back coating layer each other.
Since the magnetic layer of the ferromagnetic metal thin film magnetic recording medium has low hardness and is easily abraded, it is proposed to form, on the magnetic layer, a diamond-like carbon film and a fluorine-containing lubricant layer having a lubricity and water-repellency, successively to improve running stability and durability.
However, since a surface state of the diamond-like carbon film is very inactive chemically, when it is stored in a high temperature high humidity atmosphere, the lubricant component migrates to the back coating layer so that an amount of the lubricant on the diamond-like carbon film is reduced, and then a still-frame life is decreased.
Japanese Patent KOKAI Publication No. 353616/1992 discloses a magnetic recording medium comprising a back coating layer on a surface of a nonmagnetic substrate reverse to a magnetic layer and a fluorine-containing hard carbon film on the back coating layer to improve wear resistance of the back coating layer and also to prevent the blocking and migration of a lubricant component from a lubricant layer to the back coating layer.
However, even by the invention of the above Japanese KOKAI, it is still difficult to provide a magnetic recording medium which is excellent both in the electromagnetic conversion characteristics and durability. There are still many problems to be solved. In the structure in which the fluorine-containing hard carbon film is formed directly on the back coating layer, while a bond energy between a fluorine atom and a carbon atom in the hard carbon film is very large, an interaction between the fluorine atom and an atom on the back coating layer surface such as a carbon atom or an oxygen atom is very weak, and therefore, the fluorine-containing hard carbon film is peeled off at an interface between the hard carbon film and the back coating layer during tape running or winding, and peeled-off particles are transferred on the magnetic layer surface to cause the significant decrease of output signal or long-time clogging of the magnetic head.
Further, when a content of the fluorine atoms in the hard carbon film is large, hardness of the carbon film decreases so that the running stability of the magnetic recording medium is deteriorated.